1. Field of the Invention
The present invention relates to power supply circuits. More specifically, the present invention relates to power supply circuits and associated operating methods for power factor control.
2. Description of the Related Art
Computer systems and other electronic equipment are nearly universally powered by direct current (DC) while, for economic reasons, commercial electrical power is supplied in the form of alternating current (AC). To convert the AC power to DC power, computer systems and electronic equipment include a power supply for translating AC to DC power. Two types of power supplies are generally used to convert AC to DC power including linear power supplies and, more commonly, switching power supplies.
A linear power supply includes a transformer, one or more rectifiers, and a linear voltage regulator. The transformer reduces the voltage to a value slightly higher than the voltage required by the electronic circuits in the computer system or electronic equipment. The one or more rectifiers, usually semiconductor diodes, converts the reduced-voltage AC to DC by restricting the flow of electricity to one direction. The linear voltage regulator adjusts the voltage created by the power supply to a level suitable for usage in the computer system or electronic equipment circuits.
One example of a conventional embodiment of a switching power supply includes a silicon-controlled rectifier (SCR), a pulse width modulator, transformer, rectifier, and filtering circuit. The SCR converts the input power from a typical incoming line frequency of 60 Hz to a high frequency of about 20,000 Hz. While the SCR increases the frequency of the power signal, the pulse width modulator regulates the power signal by varying the duration of the power pulses, decreasing the pulse width to lower the output voltage. The transformer reduces the voltage to a suitable level for driving circuits. The rectifier and filtering circuit produce direct current power for usage by the computer system or electronic equipment.
Unfortunately both linear power supplies and switched power supplies introduce harmonic distortion into the power signal. For example, a switching power supply forms a nonlinear load and draws a pulse current having a waveform significantly different from the input voltage waveform. The drawn pulse current includes a fundamental current component and a plurality of harmonic current components. The fundamental current component has a waveform that matches the input voltage waveform and contributes to the power used by the power supply. However the harmonic current components contribute only to the RMS line current but not to usable power. Hence, harmonic distortion lowers the power that is available to the power supply.
A power factor is defined as the ratio of true power to apparent power. A resistive load has a current waveform and a voltage waveform that are identical and mutually in-phase. Therefore the power factor of a resistive load is equal to one, the maximum possible power factor. In contrast, the current waveform and the voltage waveform are different or out-of-phase, and the power factor is less than unity, for a load that is not purely resistive.
The high harmonic currents in a typical switching power supply leads to poor utilization of the power distribution system since the power generators of electrical utility companies need to generate RMS current even though the RMS current is not usable by the load. Furthermore, the high harmonic currents are wasteful, costly and damaging to electrical utility generating companies and electricity consumers. For example, circulating currents in the delta windings of three-phase power distribution transformers can cause temperatures in the transformers to rise to full load values before the transformers reach full load power levels. More precisely, the high energy content of the third harmonic subjects the neutral wire to an overload of 70%, possibly overloading branch circuit wiring when third harmonic current contributions from each of the three phases sum in the neutral conductor. In addition, the high harmonic currents add to stress on fuses, circuit breakers, wall sockets and wiring. The combination of a large number of personal computers or similar electronic loads operating on a common branch power circuit can distort the source-voltage wave shape considerably. An uncorrected power factor limits the output power and increases the line-current harmonics of a power supply.
As a result of the deleterious effects of harmonic distortion, governmental agencies in many nations have begun establishing standards limiting the harmonic current produced by electronic equipment and computer systems.
Various techniques have been developed to reduce harmonic distortion produced by electronic circuits. One technique for reducing harmonic distortion is to include in a power supply line a passive harmonic filter for reducing third harmonic current levels. Referring to FIG. 1, a schematic block diagram, labeled prior art, shows a parallel-connected resonant filter 102 connected between a variable utility source 104 and a power supply load RL. The resonant filter 102 compensates for nonfundamental demand and reduces the distortion component of the load current.
Also referring to FIG. 2, a schematic block diagram, labeled prior art, shows a series-connected resonant filter 202 which is useful for reducing harmonic distortions in computer systems and electronic circuits that employ a nonlinear or switching power supply. Nonlinear loads draw current discontinuously during the cycle of the input voltage waveform and produce low power factor ratio that is much smaller than the optimum unity power factor. The nonlinear loads therefore increase line current and limit the available capacity of branch circuits. The resonant filter 202 is designed to improve the power factor and decrease harmonic distortions from the power supply of a personal computer. The series-connected resonant filter 202 has an input connection to a variable utility power source 204 and an output connection to a power supply load 206 through a rectifier and filter capacitor 208.
FIG. 3, labeled prior art, shows an active boost converter circuit 302 for eliminating harmonic currents, thereby avoiding potential harmonic problems associated with the high harmonic current levels of personal computers. The active boost converter circuit 302 includes a power MOSFET 309 and operates as a switching device for eliminating harmonic distortion of the AC source current. The active boost converter circuit 302 is rated to match the power output rating of the power supply when fully loaded. The active boost converter circuit 302 has an input connection to a variable utility power source 304 through a rectifier 306. The active boost converter circuit 302 has an output connection to a variable power supply load 308. A filter capacitor C.sub.F is connected across the input lines to the variable power supply load 308. The active boost converter circuit 302 changes switch-mode power supply characteristics from nonlinear load characteristics to resistive load characteristics. The active boost converter circuit 302 generally produces a more linear load characteristic than either the parallel-connected resonant filter 102 or the series-connected resonant filter 202.